Integrated optical switching device

ABSTRACT

An integrated optical switching device includes a substrate formed with a plurality of optical input waveguides, a plurality of optical output waveguides, and a plurality of optical switching elements for selectively connecting the optical input waveguides to the optical output waveguides; characterized in that at least some of the waveguides include tight bends having a radius of curvature of less than 100 μm. Also described are compact switching devices of high port count including coaxial arrays of input and output switch columns interconnected according to a double crossbar architecture.

FIELD AND BACKGROUND OF THE INVENTION

[0001] The present invention relates to integrated optical switchingdevices, and particularly to such devices for use in high-speed datacommunication applications, such as in packet switching.

[0002] The rapid growth in transmission bandwidths of fiber-opticnetworks is enabled by, among several critical factors, enhancedswitching performance with respect to both switching speed and signalrouting. High speed and large port-count switch arrays are becomingprogressively important for high-speed data transmissions applications,in particular those supporting packet switching. Various arrayarchitectures have been developed as described for example in; R. Y.Awdeh, H. T. Mouftah: Survey of ATM Switch Architectures”,Communications Networks and ISDN systems, Vol. 27, pp. 1567-1613,November 1995 (Elvesier Science); L. Thylen, Integrated Optics in LiNbO₃ : Recent Developments in Devices for Telecommunications, Journal ofLightwave Technology, Vol. 6, No. 6, June 1988 (pages 847-861); U.S.Pat. Nos. 4,618,210; 4,787,693; and International Publication No. WO99/60434, published November 25, 1999.

[0003] However those optical switching devices that support minimumlevel of route-control processing complexity are more suitable forhigh-speed switch response. Reduced routing procedures are provided by afamily of array architectures, in particular the crossbar and itsderivative—the double-crossbar; see for example the R. A. Spankepublication cited above. Other designs are known as the DC, PILOSS,TREE. Recently an SNB (strictly-non-blocking) 16×16 switch-array basedon the TREE architecture and implemented in Z-cut LN, was reported [S.Thaniyavarn, Proceedings OFC-97, TuC1]. The TREE based device consistsof three parts: fan-out, fan-in, and a mid-section consisting of a largesilica/Si substrate, housing the connections between the 256 ports inboth the input and output mid-planes.

[0004] The above-cited International Publication No. WO 99/60434reported a recent study of a DC (Deliver and Couple) type arrayarchitecture, based on radial layout implemented Z-cut LN and TMguidance, which was shown to support a 16×16 port-count with <10 nSecreconfiguration time. The short switching-speed was aided by the factthat in only 2N switches (out of 2N² switches) are activated at each ofthe possible N! route options, and the path setting is achieved merelyby straightforward selection of the input-output ports. LN Z-cutsubstrate accommodates operation in the TM mode, independently of thepropagation angle on the substrate, due to the invariance of therefractive indexes at the propagation plane (which is not possible withX or Y cut LN). Since the major electro-optic effect operatesperpendicularly to the substrate surface in this case, the switches maybe oriented at any angle. In particular, the switches may be designedwith curvatures. While the routing procedure is much the same as that ofthe crossbar architecture (or the double-crossbar), the design has thedisadvantage of route-dependent switch paths (i.e. paths having 2 to N+1switches), which becomes a significant issue when the individual switchlosses exceed a certain level.

[0005] The double-crossbar architecture, on the other hand, supports asimilar path control procedure, and has the advantage of equalswitch-paths (always N+1). However, implementation of thedouble-crossbar by conventional waveguide elements is entirelyimpractical due to the very large number of waveguide intersections atshallow angles, which induce high losses and cross-talk levels, and inconsequence lead to excessive array length.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide an integratedoptical switching device having advantages in the above respects, andparticularly to provide such a device which enables high packing densityof waveguide elements on a single substrate. A further object of theinvention is to provide an integrated optical switching device which canbe implemented in a double-crossbar switching network.

[0007] According to one aspect of the present invention, there isprovided an integrated optical switching device comprising a substrateformed with a plurality of optical input waveguides, a plurality ofoptical output waveguides, and a plurality of optical switching elementsfor selectively connecting the optical input waveguides to the opticaloutput waveguides; characterized in that at least some of the waveguidesinclude tight bends having a radius of curvature of less than 100 μm.

[0008] According to further features in the described preferredembodiments, at least some of the tight bends define an angle ofapproximately 90° and have a radius of curvature of 20-60 μm. Such aconstruction permits very close spacing of less than 150 μm, preferablyabout 75 μm between waveguides. In addition, the 90° crossings generatevery low scattering losses and cross-talk.

[0009] According to further features in the described preferredembodiments, the tight bends, or at least some of them, are totalinternal reflection trench mirrors formed in the substrate, for exampleas described in H. Han et al, Self-Aligned High-Quality Total InternalReflection Mirrors, IEEE Photonics Technology Letters, Vol. 7, No. 8,August 1995 (pages 899-901), which publication is hereby incorporated byreference. The tight bends, or at least some of them, may also be tightridge bends formed in the waveguides.

[0010] According to another aspect of the present invention, there isprovided an integrated optical switching device, comprising: a circularwafer substrate formed with a plurality of optical input waveguides, aplurality of optical output waveguides, and a plurality of opticalswitching elements for selectively connecting the optical inputwaveguides to the optical output waveguides; the waveguides andswitching elements defining a double-crossbar switching network arrayedaccording to a circular geometry; at least some of the waveguidesincluding tight bends having a radius of curvature of less than 100 μmand spaced from adjacent waveguides by a space of less than 150 μm.

[0011] According to further features in the described preferredembodiments, the double-crossbar switching network includes a firstcircular array of input switch columns defining optical switchesinterconnected according to a first crossbar architecture, and a secondcircular array of output switch columns defining optical switchesinterconnected according to a second crossbar architecture; the secondcircular array of output switch columns being coaxial with, andconnected to outputs of, the first array of input switch columns.

[0012] In the described preferred embodiments, the second circular arrayof output switch columns is located on the substrate outwardly of thefirst circular array of input switch columns.

[0013] The foregoing features enable integrated optical switchingdevices to be constructed having a number of important advantages overthe prior art constructions briefly described above.

[0014] A particularly important advantage is that these features enableoptical switching devices to be constructed according to the DCB(double-crossbar) architecture. Thus, by employing either 90° reflecting3D mirrors, or tight ridge-waveguides bends, and also by implementing aradial geometry, the DCB architecture yields a superior packing-factor(i.e. number of switches or array dimensions) compared to the DC(delivery and couple) architecture. The 3D mirrors may be fabricated bythe high precision “RIE” (reactive ion etching) process to provide highfacet quality. Such 3D mirrors have previously been incorporated incommercial processing of laser diodes based on III-V materials (OliverGraydon, Opto-Laser Europe, October 97, p. 11), but not in opticalswitch-arrays. Etched surfaces are employed for 90° reflection inoptical switch-arrays. The 45° facet-angle provides fortotal-internal-reflection (TIR) if the two materials index-ratio >1.4(in the LN case the ratio is 2.14 and in the case of silica waveguidesit is 1.45).

[0015] Alternatively, tight ridge-waveguide bends may also be employed.In such case a similar effect is obtained and also the fabrication iseasier.

[0016] The proposed layout is optimal in most aspects of connection pathefficiency, except for the different number of crossings in thedifferent paths. However, in this case, this is a relatively minormatter since waveguides intersecting at ˜90° have practically negligiblelosses (when ΔN<<N, as is the case in LN substrates, where ΔN is thewaveguide index-perturbation).

[0017] In addition to providing for better area utilization, and therebyhigher packing density, as compared to the DC architecture, theforegoing features permit improved processing uniformity, as compared toother designs, thereby also improving the pattern accuracy as well asthe uniformity of the switching voltages (between the array's switches).

[0018] According to still further features in one preferred embodimentdescribed below, the input waveguides include a plurality of input rows,with each input row including a splitter for splitting the respectiveinput row into a plurality of branches (preferably two); and the outputwaveguide includes a combiner for each plurality of the branches forcombining them into their respective output waveguides. In this setupthe “maximum cross-talk figure” of any of the array's outputs is reducedmarkedly (from a maximum of ˜[N−2]X² to ˜[N/2−1]X³, where X is thefractional cross-talk of the switch, and N is the number of the arraycolumns/rows). As an example, the maximum cross-talk figure of theswitch array will be the same when the switch cross-talk is −20dB in theformer design and 13.5dB in the design discussed here. This allows therelaxation of the critical design parameters of the individual switch,while the overall performance improves.

[0019] As indicated above, the invention is particularly useful inapplications wherein the switching elements define a DCB(double-crossbar) switching network. In one described embodiment, thesubstrate is a three-inch circular LN wafer, and the DCB switchingnetwork includes an array of 16×16 to 20×20 switching elements arrayedaccording to a circular geometry; in a second described embodiment, thesubstrate is a four-inch circular LN wafer, and the DCB switchingnetwork includes an array of 16×16 to 24×24 switching elements arrayedaccording to a circular geometry; in a third described embodiment, thesubstrate is a five-inch circular LN wafer, and the waveguides andswitching elements define a DCB switching network of up to 32×32switching elements. Such packing densities have heretofore not beingobtainable insofar as the inventor is presently aware (on a singlesubstrate, and while supporting the strictly-non-blocking property).

[0020] Preferably, the substrate is a Z-cut lithium niobate (LN)substrate. However, the invention could be implemented as well withother substrate materials. A most attractive option is the electro-opticpolymer (EO-polymer) waveguide patterned on a silicon substrate (orother substrates such as quartz). A particularly attractive option isthe silica/electro-optic polymer (EO-polymer waveguide composite (orsimilarly silicon oxy-nitride instead of silica), patterned on a siliconsubstrate. By this method, as an example, the EO polymer is patternedbetween the silica waveguides at the switch zone. The silica waveguidespresently have the lowest propagation losses. With adequate switchdesign, light propagates predominantly in the low loss silica waveguidesat the straight through state, and permeates the higher loss EO polymermedium when at the cross state. Since the refractive indexes of silicaand EO-polymer are nearly the same, the transition between the twomediums is virtually without loss. Silica waveguides are easily designedfor tight bends or alternatively can be easily etched to provide highquality 45° TIR mirrors. Silica or polymer waveguide technologies can bepatterned on substrates with diameters as large as 6 or 8 inches,potentially supporting array designs with port counts higher than 32×32on a single substrate. However, in practice, the choice of the substratematerials depends on the propagation loss, electrical power consumption,and finally pattern uniformity, according to the processing technologyavailable at the time.

[0021] According to still further features in another preferredembodiment described below, the substrate is of a rectangular shape andthe waveguides in the arrays are patterned accordingly, in a rectangularloop. This embodiment may be particularly useful with silica or polymerwaveguide technologies, and provides improved utilization of thesubstrate area.

[0022] Further features and advantages of the invention will be apparentfrom the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

[0024]FIG. 1 diagrammatically illustrates an array of waveguides andswitching elements defining a double-crossbar (DCB) switching network;

[0025]FIG. 1a diagrammatically illustrates one form of tight bend (TB)in the switching network of FIG. 1, namely one in the form of a kneebend with a total internal reflection TIR) trench mirror;

[0026]FIG. 1b diagrammatically illustrates another form of tight bend(TB) in the switching network of FIG. 1, namely one in the form of atight ridge formed in the waveguide;

[0027]FIG. 2 diagrammatically illustrates a DCB switching networkconstructed in accordance with the invention in which the waveguides andswitch elements are arrayed according to a circular geometry, theexample illustrated being a 2×20×20 array implemented on a three-inchcircular wafer;

[0028]FIG. 2a is an enlarged fragmentary diagram illustrating theelectrical and waveguide connections between two adjacent switch columnsin area “a” in the wafer of FIG. 2;

[0029]FIG. 3 illustrates another DCB switching network in the form of acircular DCB architecture with two concentric bands, constructed inaccordance with the invention, implemented on a four-inch circular waferand containing a 2×32×32 array;

[0030]FIG. 4 illustrates a 4×4 DCB array with split rows showing onemanner in which each input row waveguide may be split into two (or more)branches and the branches combined in the output waveguide;

[0031]FIG. 5 illustrates a circular architecture of the DCB arrayshowing one manner in which the splitting/combining technique of FIG. 4may be implemented in a five-inch circular wafer to having a 2×32×32array; and

[0032]FIG. 6 illustrates a 2×32×32 DCB array with rectangular geometryas an example of a switching network constructed on a rectangularsubstrate where the arrays are patterned according to a rectangulargeometry.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0033]FIG. 1 diagrammatically illustrates a plurality of waveguides andswitching elements defining a double-crossbar (DCB) switching network.Thus, as shown in FIG. 1, a first group of waveguides and switchingelements are arranged to define a first crossbar (CB) indicated by boxCB₁ serving as an input array IA, and another group of waveguides andswitch elements are arranged to define a second crossbar, indicated bybox CB₂ connected to the output ends of the crossbar CB₁, of the inputarray, IA, and serving as an output array OA. The waveguides of theinput array IA are connected by tight bends TB to the waveguides of theoutput array OA. Each tight bend TB may be as shown in FIG. 1a or FIG.1b, to define an angle of approximately 90° and to have a radius ofcurvature of less than 100 μm, preferably of 20-60 μm.

[0034] As shown in FIG. 1, each of the electro-optical switches EOS inthe input array IA is of the 1×2 type, wherein a signal appearing in aninput waveguide may be selectively passed through the interaction zoneto appear in the same waveguide at the output, or crossed in theinteraction zone to appear in an adjacent waveguide. On the other hand,each of the electro-optical switches EOS in the output array OA is ofthe 2×1 type, wherein the signal on either of two input waveguides iscontrolled to pass through the interaction zone into the outputwaveguide.

[0035] The switching array illustrated in FIG. 1 is of a SNB (StrictlyNon-Blocking) architecture, and includes a 4×4 array, requiring 2×4×4,or 32, electro-optical switches EOS. That is, each path requires theactivation of two switches, such that for any routing configuration, 2×Nswitches are to be activated out of the 2×N×N. Such an array preventsthe occurrence of conflicting paths, and supports broadcast andmulticast.

[0036] Further details of the operation of such DCB (double-crossbar)switching networks are described in the literature, for example in theabove-cited 1987 publication of R. A. Spanke, incorporated herein byreference. However, as described earlier, the double-crossbar switchingnetwork diagrammatically illustrated in FIG. 1 was, as a practicalmatter, heretofore impossible to be implemented on an integrated opticalwafer because of the very large number of waveguide intersections atshallow angles, which induce high losses and cross talk noise, and inconsequence, lead to excessive array lengths.

[0037] According to the present invention, such a DCB switching networkis implemented by providing the waveguides, or at least some of them,with the tight bends shown at TB in FIG. 1. Each of the tight bends TBhas a radius of curvature of less than 100 μm, preferably in the orderof 20-60 μm. FIGS. 1a and 1 b illustrate two manners of producing suchtight bends.

[0038] Thus, FIG. 1a illustrates the tight bend TB between the twowaveguides in the form of a 90° waveguide knee bend with a totalinternal reflection (TIR) trench mirror TRM formed in the substrate. Theabove-cited publication of 1995 by H. Han et al, incorporated herein byreference, describes one manner of fabricating such a TIR mirror using areactive ion etching (RIE) technique.

[0039]FIG. 1b illustrates another manner of forming such a tight bendTB, wherein the waveguide is formed as a tight ridge bend in thesubstrate, by etching the regions ER surrounding the bend. The bendradius could be in the order of tens of μm. Preferably, a substantially90° bend is formed with a radius of 20-60 μm. The bend losses due toradiation decrease with increased confinement degree. In LN waveguides,the confinement degree is very high because of the large indexdifference of the substrate and the air (2.15 vs 1). This is to bedistinguished from other waveguides wherein the index contrast is verysmall, in which case typical low loss radii are larger by several ordersof magnitude, e.g., radii of millimeters or tens of millimeters.

[0040] As described more particularly below, providing such tight bendsTB produces a number of important advantages in integrated-opticalswitching devices in general, which advantages are particularlysignificant in DCB (double-crossbar) switching networks arrayedaccording to a circular geometry. Thus, such tight bends enable thespacings between the matrix rows to be extremely small, in the order of75-150 μm, thereby permitting extremely high-packing density of opticalswitches on a single substrate. In addition, implementing the network ina circular format on a Z-cut LN substrate is permitted due to thenon-limited angular orientation of the waveguides and switches whenoperating with the TM polarized optical mode. The resulting combinedadvantages are optimum radial symmetry, which also improves processinguniformity and therefore fabrication yield, and a high level ofuniformity in the switching voltages.

[0041]FIG. 2 illustrates the invention implemented in a three-inchcircular wafer formed with a DCB (double-crossbar) switching networkincluding a 2×20×20 array according to a circular geometry. As shown inFIG. 2, each of the 20 switch columns in the input array IA includes aninput crossbar CB₁ at the periphery of the circular wafer, and an outputcrossbar CB₂ also at the periphery of the wafer but outwardly of theinput array crossbar CB₁. Crossbars CB₁, are interconnected to define acircular input array IA, and crossbars CB₂ are interconnected to definea circular output array OA coaxially with, and outwardly of, the inputarray IA.

[0042] As further shown in FIG. 2, the substrate further includes aninner array of electrical pads IEP arranged in a rectangular matrix ontop of the substrate. Each pad is connected by a lead (not shown) to oneparticular switch in the array. The pads form the electrical contactpoints to the external control circuit, serving for selectivelyactuating the optical switches defined by the waveguides in thecrossbars CB₁, CB₂ of the input and output arrays IA, OA, respectively.Pads IEP are connected to the external control-circuit by conventionalwire bonds or alternatively by an overlying PCB (not shown). Theillustrated substrate further includes a rectangular array of outerelectrical pads OEP, outwardly of the waveguide crossbars CB₁, CB₂,these pads being accessed as well either by conventional bondingtechnology or by an overlying PCB.

[0043]FIG. 2a is an enlarged fragmentary view of region “A” in FIG. 2,more particularly illustrating the electrical and optical connectionsbetween the input array crossbar CB₁ and output array crossbar CB₂ intwo adjacent switch columns.

[0044]FIG. 2a particularly illustrates a waveguide WC with two 90°bends, connecting between the crossbar CB₁ of the input array IA, andthe crossbar CB₂ of the output array OA, as well as the electricalconnections EC from the electro-optical switches EOS to the pads (of theindicated switched path). The entire path from input to output,traverses four 90° tight bends (e.g., reflections or tight ridge bends),and the total propagation path is approximately 200 mm.

[0045] Considering accepted values with the present technologyprocessing techniques of 1.5 dB loss per tight bend by reflection, 0.1dB/cm propagation loss, 0.5 dB coupling loss at the substrate interface,and negligible switch losses, the expected insertion losses would amountto approximately 9 dB. Even this would be acceptable in thisapplication, but it is expected that improved processing techniques willeven further reduce the insertion losses to about 6 dB.

[0046] Thus, a Z-cut LN three-inch wafer, with 100 μm spacing betweenthe array rows, and with 4 mm spacing between the wafer perimeter andthe external array row, can support an array of 16×16 to 20×20 switches,with the individual switch length of 6 mm. A properly designed 6-mm longswitch can be switched with an amplitude of less than 15V. Electrodes atthis length, with typical capacity range of ˜5 pF, can alternate betweenswitch states at <10 nSec by employing state-of-art electronics. Basedon the above parameters, as an example in a 20×20 array of switches, thewaveguide-array would be 4 mm in width, occupying a radial zone betweenR=29 mm and R=33 mm, and would have curvatures that have been provedpreviously to support sufficiently low propagation losses [E. Voges et.Al, True time delay integrated optical RF phase shifters in lithiumniobate, Elec. Lett., Vol. 33, No. 23, pp.1950-51, November 97]. Bothfactors, i.e. the array-pattern confinement in a narrow radial width, aswell as the radial layout in itself, contribute to the fabricationuniformity, and therefore to the enhancement of the lithographic yieldfactors, in terms of both overall registration accuracy, and low faultprobability.

[0047] Whereas the described construction can be implemented on athree-inch wafer to support an array of 16×16 to 20×20 switchingelements, particularly a 2×20×20 array, this same construction can beimplemented on a four inch Z-LN substrate to support an array of 16×16to 24×24 switching elements, particularly a 2×24×24 array, withindividual switch lengths of 7 mm. In addition, a considerably largerport count can be supported by the same substrate by splitting eachannular array into two concentric ribbon of row bundles to form an arrayband.

[0048] Thus, FIG. 3 illustrates the invention implemented in a four-inchwafer formed with a double-crossbar switching network including a2×32×32 array in two concentric ribbons or bands. As an example, a2×32×32 array could be split into outer and inner interconnected annularbands (or row bundles) of 18 and 14 columns, respectively, with eachcolumn including 32 switches, and with an individual switch length of 7mm. The innermost row in the inner ribbon (or row bundle) would have adiameter of 64 mm, thereby supporting very low curvature losses. Theinsertion loss is estimated to be of the order of 10-14 dB. Theelectrodes of the inner annular ribbon (or row bundle) are accessedelectrically from the substrate's center zone, IEP, while those of theouter ribbons would be accessed electrically from the outer electricalpads OEP or by an overlying PCB.

[0049]FIG. 4 diagrammatically illustrates a technique using splitters Sand combiners C particularly for switching networks having a largematrix of switching elements to occupy a relatively small-diameterwafer. Such a technique is particularly useful for providing a matrix of2×32×32 switching elements arrayed in concentric circles on a four-inchwafer, as compared to a maximum of 2×24×24 using the design of FIG. 3.

[0050] Briefly, this technique involves: (1) splitting each of the inputrows to a plurality (two) of branches at the input access; (2) combiningthe output rows in reversed fashion; and (3) increasing the number ofelectrical and optical connections to accommodate the abovemodification. This technique enables obtaining a 2×32×32 array on a fourinch or five-inch Z-cut LN substrate.

[0051]FIG. 4 illustrates four input rows 41-44 coupled to four outputrows 51-54, respectively. The coupling is accomplished with only twoswitch columns, instead of the four in the conventional DCB(double-crossbar) scheme, and with eight rows, which are actually foursplit rows. An N×N switch will require 2×N×N switches as previously, butalso an additional 2×N switches for splitting at splitters S₁-S₄ andcombining at combiners C₁-C_(4,) respectively.

[0052] By thus doubling the number of the rows, the number of thecolumns is halved, and therefore the effective switch column number isincreased per given array length.

[0053]FIG. 5 diagrammatically illustrates an implementation of thesplitting/combining technique of FIG. 4 applied to form, on a four-inch(or a five-inch) circular wafer, a DCB (double-crossbar) switchingnetwork including a 2×32×32 array. Preferably, the splitting andcombining switch columns (SA and CA) are located at the central zone ofthe wafer, so that all the circular section is available for theswitching columns.

[0054] The splitting/combining technique of FIG. 4 has the additionaladvantage of providing for reduced average crosstalk: Half of thepossible paths in the switch array include the combining and splittingswitches set at OFF state. The optical-power leakage from this half intothe selected paths is reduced by a factor of X², where X is thefractional crosstalk of the switch.

[0055] Table I below sets forth various designs that may be implementedon Z-cut LN substrate in accordance with the above-described features:TABLE I Row Total switch Minimum Substrate Port Array Switch separationnumber curvature diameter count bands length [mm] [μm] 2 × N × N radius[mm] 3″ 16 1 8.5 100 512 30.0 3″ 20 1 6.2 100 800 28.5 4″ 20 1 9 100 80041.5 4″ 24 1 7.4 100 1152 40.5 4″(a) 32 2 8.1 100 2048 31.5 (14/18)5″(b) 32 1 9 100 2048 45

[0056] (a) array architecture of two concentric bands (FIG. 3)

[0057] (b) array architecture of split rows (FIG. 4)

[0058]FIG. 6 diagrammatically illustrates an implementation of the sameDCB architectures described in FIGS. 1-5 on a rectangular substrate.More particularly, it illustrates a 2×32×32 DCB array with rectangulargeometry. This layout may be preferred when employing silica or polymerwaveguides.

[0059] The described switch-array concept may be implemented, ingeneral, with all known substrate materials that support TIR trenchmirrors of adequate quality, or tight ridge waveguide bends. Fabricationmay be done by a routine fabrication sequence, e.g., as follows: (1)pattern the waveguides; (2) apply the waveguide cladding; (3) patternthe electrodes; (4) fabricate the knee-bends/ridge-bends; (5) package ina box; (6) make the electrical connections (by conventional wire-bondingand/or flip-chip contacting); and (7) make the fiber ribbon connections(“pigtailing”).

[0060] The electrical leads from the electrodes to the pads, in theparticular radial switch design, extend either into the substrate'scenter or to the substrate's perimeter (likely to both zones).Alternatively, the pads could be designed to share the electrode zonesadjacent to the waveguides, and the electrical interfacing is achievedby employing the “flip chip” technology. By the latter option, spacingbetween the switch-columns otherwise occupied by the leads, according tothe first option, can be used for increased switch lengths (andtherefore for improved switch performance). The later method is alsomore efficient for interfacing large number of electrical ports with thePC (printed circuit) board.

[0061] In the above description, the examples referred to arraydimensions of N×N. However, the Double-Crossbar architecture applies aswell to array dimensions of N×M, where M≠N, N and M are the number ofinput and output ports, respectively.

[0062] While the invention has been described with respect to a numberof preferred embodiments, it will be appreciated that these are setforth merely for purposes of example, and that many other variations,modifications and applications of the invention may be made.

What is claimed is:
 1. An integrated optical switching device,comprising a substrate formed with a plurality of optical inputwaveguides, a plurality of optical output waveguides, and a plurality ofoptical switching elements for selectively connecting said optical inputwaveguides to said optical output waveguides; characterized in that atleast some of said waveguides include tight bends having a radius ofcurvature of less than 100 μm.
 2. The device according to claim 1,wherein at least some of said tight bends define an angle ofapproximately 90°.
 3. The device according to claim 1, wherein at leastsome of said tight bends have a radius of curvature of 20-60 μm.
 4. Thedevice according to claim 1, wherein the spacing between at least someof said waveguides is less than 150 μm.
 5. The device according to claim1, wherein the spacing between at least some of said waveguides is about75 μm.
 6. The device according to claim 1, wherein at least some of saidtight bends are total internal reflection trench mirrors formed in thesubstrate.
 7. The device according to claim 1, wherein at least some ofsaid tight bends are tight ridge bends formed in the waveguides.
 8. Thedevice according to claim 1, wherein said substrate is a Z-cut lithiumniobate substrate.
 9. The device according to claim 1, wherein saidsubstrate is of circular configuration, and said waveguides and switchelements are arrayed according to a circular geometry.
 10. The deviceaccording to claim 1, wherein said waveguides and switching elementsdefine a double-crossbar switching network.
 11. The device according toclaim 10, wherein said double-crossbar switching network includes afirst array of input switch columns defining optical switchesinterconnected according to a first crossbar architecture, and a secondarray of output switch columns defining optical switches interconnectedaccording to a second crossbar architecture; said second array of outputswitch columns being coaxial with, and connected to outputs of, saidfirst array of input switch columns.
 12. The device according to claim11, wherein said second array of output switch columns is located onsaid substrate outwardly of said first array of input switch columns.13. The device according to claim 11, wherein said substrate furtherincludes electrical connections to said first and second arrays ofswitch columns located both inwardly of and outwardly of said first andsecond arrays of switch columns.
 14. The device according to claim 11,wherein said first and second arrays are circular arrays.
 15. The deviceaccording to claim 1, wherein said first and second arrays arerectangular arrays.
 16. The device according to claim 11, wherein saidsubstrate includes a single band of said first and second arrays. 17.The device according to claim 11, wherein said substrate includes aplurality of concentric bands each including said first and secondarrays.
 18. The device according to claim 11, wherein said substrate isa three-inch circular wafer, and said double-crossbar switching networkincludes an array of 16×16 to 20×20 switching elements arrayed accordingto a circular geometry.
 19. The device according to claim 11, whereinsaid substrate is a three-inch circular wafer, and said double-crossbarswitching network includes 2×32×32 switching elements arranged in twoconcentric bands according to a circular geometry.
 20. The deviceaccording to claim 11, wherein said substrate is a four-inch circularwafer, and said double-crossbar switching network includes an array of16×16 to 2×24×24 switching elements arrayed according to a circulargeometry.
 21. The device according to claim 11, wherein said substrateis a five-inch circular wafer, and said double-crossbar switchingnetwork includes an array of up to 32×32 switching elements arrayedaccording to a circular geometry.
 22. The device according to claim 1,wherein said input wave guides include a plurality of input rows, witheach input row including a splitter for splitting the respective inputrow into a plurality of branches; and wherein said output waveguideincludes a combiner for each plurality of said branches for combiningthem into the respective output waveguide.
 23. The device according toclaim 22, wherein each input waveguide is split by said splitter intotwo branches, and wherein each combiner combines the two branches intothe respective output waveguide.
 24. The device according to claim 23,wherein said substrate is a four-inch circular wafer, and saidwaveguides and switching elements define a double-crossbar switchingnetwork of 2×32×32 switching elements.
 25. The device according to claim23, wherein said substrate is a five-inch circular wafer, and saiddouble-crossbar switching network includes an array of 2×32×32 switchingelements arrayed according to a circular geometry.
 26. An integratedoptical switching device, comprising: a circular wafer substrate formedwith a plurality of optical input waveguides, a plurality of opticaloutput waveguides, and a plurality of optical switching elements forselectively connecting said optical input waveguides to said opticaloutput waveguides; said waveguides and switching elements defining adouble-crossbar switching network arrayed according to a circulargeometry; at least some of said waveguides including tight bends havinga radius of curvature of less than 100 μm and spaced from adjacentwaveguides by a space of less than 150 μm.
 27. The device according toclaim 26, wherein said double-crossbar switching network includes afirst circular array of input switch columns defining optical switchesinterconnected according to a first crossbar architecture, and a secondcircular array of output switch columns defining optical switchesinterconnected according to a second crossbar architecture; said secondcircular array of output switch columns being coaxial with, andconnected to outputs of, said first array of input switch columns. 28.The device according to claim 26, wherein said input waveguides includea plurality of input rows, with each input row including a splitter forsplitting the respective input row into two branches; and wherein saidoutput waveguide includes a plurality of output rows, with each outputrow including a combiner for combining said two branches into theirrespective output waveguides.
 29. The device according to claim 1,wherein said substrate is of silicon, said waveguides are of silica, andsaid switching elements include switch interaction zone of anelectro-optic polymer.
 30. The device according to claim 1, wherein saidsubstrate is of silicon, said waveguides are of silicon oxy-nitride, andsaid switching elements include switch interaction zone of anelectro-optic polymer.